This invention relates to amplified optical fibre systems and in particular to amplified long haul telecommunications systems especially soliton systems.
In an ideal soliton system, a pulse propagates down a fibre and suffers no dispersion since there is a dynamic balance between the new frequency components developed by self-phase modulation and the negative group velocity dispersion. The negative dispersion causes the newly generated high frequency components to speed up relative to the lower frequency components thereby causing the pulse to retain its shape. Unfortunately, "ideal" conditions require that the transmission line (fibre) is lossless i.e. zero attenuation. Since the losses in a fibre are finite, some form of optical amplification is required to overcome the losses. With optical amplifiers it is possible to boost a pulse, which has reached the point where it is losing its soliton characteristics, back to being a soliton again. The choices of amplifiers are semiconductor laser, erbium fibre and Raman fibre amplifier. The most common currently being the semiconductor laser and erbium fibre. Both of these amplifiers are "lumped" amplifiers, that is to say they are discrete devices (a diode amplifier or ten metres of erbium fibre) spliced into the transmission fibre at regular intervals. Thus, although the fibre loss affecting a transmitted pulse can be offset by the gain of the amplifier, the transmission line cannot be described as lossless since zero loss only occurs at one point in an amplifier span.
The problem for soliton pulses in such systems is that the peak power will vary down the length of the fibre. If the peak power exceeds a value of 9P/4 where P is the optimum power for the lowest (first) order soliton, then a higher order soliton will be produced which is unsuitable for data transmission since it contains multiple peaks and changes shape as it travels down the fibre. On the other hand if the peak power in the soliton falls below P/4, the conditions for soliton transmission are violated and the pulse will simply spread out due to dispersion and the soliton is lost. Thus the distance between the amplifiers in a soliton system is going to be determined by these power limits (9P/4 to P/4) and for very long haul high bit-rate systems the amplifier spacing was thought to be restricted by the Gordon-Haus effect to about 25 to 30 Km, for shorter haul systems of say a few hundred kilometers wider amplifier spacings of 50 or more Km may be obtainable. This is actually worse than conventional non-soliton system repeater spacings, although the optical amplifier approach is such that, if it can be done at 25 Km spacings, then it can be done at 25 Km spacings for thousands of kilometres, which is not possible with ordinary intensity modulation. These spacings may need to be reviewed since there is a current thought that the Gordon-Haus effect may be avoidable using filters. The Gordon-Haus effect is a phenomenon in which non-linear mixing between the signal and amplified spontaneous emission in a long multi-amplifier soliton system causes random frequency shifts in the pulses, thereby producing delay shifts eventually leading to inter-symbol interference. Gordon and Haus derived an expression which defines the limits for spacing of amplifiers and the overall system length before these effects cause signal impairment.
We currently consider that it will not be possible to achieve wider amplifier spacings, say 80 to 100 Km, with lumped amplifiers even if the latter could be improved. The main reason for this is that the soliton power has to lie between P/4 and 9P/4 for a first order soliton and whilst the pulse may spread out, provided its power does not fall below P/4 it can be boosted back to being a first order soliton again, although the power to which it is boosted must then not exceed 9P/4. Hence the repeater spacing for lumped amplifiers is determined by how much the power drops before the soliton is lost.
The above discussion is concerned with the soliton propagation regime in which the soliton period Z.sub.o is small compared with the amplifier spacing and the power is kept within the limits mentioned. ##EQU1## where T is the pulse width, .lambda. is the wavelength and D is the fibre dispersion.
Soliton systems are designed for use at 1.55 .mu.m and with conventional fibres having a zero dispersion wavelength .lambda..sub.o of 1.3 .mu.m, the dispersion at 1.55 .mu.m is sufficiently large that the soliton period is typically a few kilometers.
There is, however, now another soliton propagation regime i.e. one which uses dispersion shifted fibre, in which case the soliton period is at least an order of magnitude longer because the dispersion is smaller. A dispersion shifted fibre is a single mode fibre in which the wavelength of zero dispersion has been shifted out of the 1.5 .mu.m low loss window. This is achieved by having a higher delta n and/or smaller core size than conventional (1.3 .mu.m) fibre. Given that typical spacings between amplifiers is 30 Km, this other regime is one in which the soliton period is much longer than the amplifier spacing. It has been demonstrated that in this regime the soliton power can be much larger than the 9P/4 level refered to above. Indeed this other regime, known as the average soliton regime, as long as the average power over one amplifier period is equal to the N=1 soliton power, the soliton propagates stably. In other words, the soliton is insensitive to perturbations in power or fibre dispersion, provided the average soliton power rule is not violated.
The present invention aims to provide means whereby wider amplifier spacings than hitherto can be achieved.